1. Field of the Invention
The present invention relates generally to assembly line arrangements for producing vehicles, and more particularly, is directed &o an improvement in an arrangement for a vehicle assembly line in which a vehicle body is conveyed to be provided with various parts mounted thereon.
2. Discription of the Prior Art
There has been previously proposed a vehicle assembly line arrangement including a body assembly line in which a vehicle body fed thereinto with doors after having passed through a coating process is conveyed to be subjected to a body assembling process and a door subassembly line, formed into an individual circular assembly line, in which the doors detached from the vehicle body on the body assembly line are conveyed to be subjected to a door assembling process, as disclosed in, for example, the Japanese patent application published before examination under the publication No. 59/57075 on Apr. 2, 1984. In such a vehicle assembly line arrangement proposed previously, each of the doors accompanying the vehicle body fed into the body assembly line is detached from the vehicle body at a door removing station located in the body assembly line and guided to the door subassembly line in which various door parts are mounted on the door, and then each door provided fully with the door parts is transferred from the door subassembly line to a door mounting station located in the body assembly line to be mounted to the corresponding vehicle body without the doors, which is provided with various body parts during its movement along the body assembly line. The door subassembly line is provided with a door conveying device which is usually formed into an overhead conveyor comprising an overhead circular rail and a plurality of door hangers each engaged with the overhead circular rail. Each of the door hangers moves to circulate along the overhead circular rail and holds the door detached from the vehicle body at the door removing station in the body assembly line so as to transport the same along the door subassembly line from the door removing station to the door mounting station.
In the case of the arrangement mentioned above, the door removing station and the door mounting station are located to have a predetermined relatively long distance therebetween so that the vehicle body is able to be provided fully with the body parts during its movement along the body assembly line from the door removing station to the door mounting station. In connection with this, it is required for the door conveying device in the door subassembly line to have a relatively long distance between a portion thereof positioned at the door removing station and another portion thereof positioned at the door mounting station, so that the time lapsed during the movement of the door made along the door subassembly line from the door removing station to the door mounting station is equal to the time lapsed during the movement of the vehicle body made along the body assembly line from the door removing station to the door mounting station.
Under such a situation, since the time lapsed during an actual door parts mounting period in which each door is provided fully with the door parts in practice after being detached from the vehicle body is relatively shorter than the time lapsed during the movement of the door made along the door subassembly line from the door removing station to the door mounting station, many doors provided fully with the door parts are to be conveyed slowly toward the door mounting station by the door conveying device especially at the rear half of the door subassembly line. Accordingly, the door conveying device is required to have a large number of door hangers and the overhead circular rail by which the door hangers are supported is burdened with a very heavy load, and consequently the overhead circular rail, a housing structure supporting the overhead circular rail and other members constituting the door conveying device are necessitated to be constructed firmly enough. This results in a problem that the expense of facilities are considerably increased, and also in a disadvantage that the door hangers increased in number make noise of large magnitude.
In this connection, in comparison with the weight of the door without the door parts, the weight of the door provided fully with the door parts is increased by, for example, about seventy-five percent, such as to thirty five kilograms from twenty kilograms.